Sorry, completely missed your last question, Gurnk! Basically it's all down to the JOINs:

1. Join picture.id to picture_categories.id based on WHERE clause (one to many);
2. Join picture_categories.category_id to categories.id (many to many);

Key:
Blue = Table
Green = Column

So for every ID that matches to the picture's image in our lookup table, get all the IDs that match up to a categories in the categories table.

__________________
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.